About HypoGen Cost analysis.

Generally speaking, the Null Cost should larger than the Fixed Cost. The larger the difference between the two values, the better the results. However, I found the Null is a little bit smaller than the Fixed when I using HypoGen with 24 molecules!!! Their IC50 is form 9 to 185 nmol. How could I meet this problem? Anybody can help me? Thanks in advance!